1. Field of the Invention
This invention relates to precision measurements of large and small displacements and particularly relates to an absolute distance measuring interferometer (ADMI) which measures the absolute distance between the two mirrors of a multi-pass resonant cavity that is referenced to a frequency-stabilized laser source.
2. Description of Related Art
The capability of measuring large and small distances with high precision is useful in a broad range of applications including those within the field of metrology, high-resolution X-ray lithography, and optical lithography. Conventional high-resolution distance measuring interferometers measure only relative changes in distance. This measurement is accomplished by comparing the distance to be measured with some calibrated distance such as the wavelength of a known frequency of light. Conventional interferometers carry out this measurement by actually counting the number of wavelengths while the change in distance is being made; consequently, they can only measure relative changes in distance.
A conventional high-resolution interferometer consists of a laser source which generates a coherent source of monochromatic radiation of wavelength .lambda.. The output of the laser is split by a beamsplitter to enter the two arms of an interferometer. One of the arms is the reference arm while the other is the measuring arm of the interferometer. Both arms have a high reflectivity mirror to retro-reflect the light directly back to the beamsplitter. When the light reflects off the mirrors in both arms and is re-combined at the beamsplitter, the combined light is directed toward a detector. The light incident on the detector can undergo constructive or destructive interference due to the coherent properties of the laser radiation. Whether or not the emerging beam undergoes constructive or destructive interference depends on the relative difference in length of the two arms of the interferometer. If the measuring arm changes in length by one wavelength, then the emerging beam will undergo a single cycle of changing from constructive to destructive interference and back again.
One can measure changes in the length of the measuring arm of the interferometer by counting the number of cycles of constructive/destructive interference and hence the number of wavelengths of light while the measuring mirror moves through that range. This interferometer must remain carefully aligned during the motion to ensure that an accurate counting of the wavelengths is achieved. If the alignment is disrupted during the measurement, then all the relevant information is lost.
There are existing systems which interferometrically measure absolute distances, but they suffer from problems of complexity, size, or lack of dynamic range. For example, U.S. Pat. No. 5,412,474 to Reasenberg et. al. describes a system in which the distance is measured through a determination of the Free Spectral Range (FSR). This system requires two probe frequencies and multiple feedback systems in order to lock the laser source to two successive maxima and/or minima (nodes) in the transmission through an optical cavity. This system also requires that the cavity to be measured must have a length that is a half-integer multiple of the laser wavelength. This requires that the laser wavelength be shifted to match the cavity or that the cavity length be altered to match the laser.